Fuel injection systems typically employ multiple fuel injectors to deliver injections of high pressure fuel into an engine for combustion. Each fuel injector typically includes a nozzle assembly having a pressurized chamber configured to contain a volume of pressurized fuel. During an injection, the volume of pressurized fuel is expelled through an orifice within the nozzle assembly. Typically, injectors also include a needle valve element that is slidably disposed within the pressurized chamber. The needle valve element may be biased by a spring towards a closed position where the orifice is blocked. To inject fuel, the needle valve element is selectively moved to unblock the orifice, thereby allowing high pressure fuel to flow from the pressurized chamber, through the orifice, and into the engine.
Selective movement of the needle valve element may be controlled by a control valve and a control chamber. The control chamber may be selectively filled and drained of pressurized fuel. When the control chamber is full of pressurized fuel, the fuel may act on a hydraulic surface of the needle valve element and bias the needle valve element into the closed position, thereby closing the injector. To open the injector, a piezoelectric actuator may move the control valve and release the pressurized fuel within the control chamber into a drain. Depressurization of the control chamber causes a change in the bias of the needle valve element and, therefore, encourages the needle valve element to move into the unblocked position. The piezoelectric actuator typically consists of a piezoelectric stack having multiple layers of piezoelectric material separated by electrically conductive layers that act as electrodes. When an electrical potential is applied across the electrically conductive layers, the piezoelectric stack expands longitudinally. The longitudinal expansion provides the motion necessary to move the control valve and depressurize the control chamber. Although this configuration may be effective for initiating the injection of fuel, the piezoelectric stack is brittle and may crack if it is overloaded.
One method utilized by injector manufacturers to isolate the piezoelectric stack from damaging forces is described in U.S. Pat. No. 7,145,282 (the '282 patent) issued to Oakley et al. The '282 patent describes a fuel injector for an internal combustion engine, including a piezoelectric actuator having a piezoelectric stack isolated within a casing. The casing is rigid and is designed to absorb shear stresses. Further, the case is configured to place a preload on the piezoelectric stack. which may protect the piezoelectric stack by keeping the piezoelectric stack in compression even when tensional forces are acting on the actuator.
Although the piezoelectric actuator of the '282 patent may be protected from shear forces and tensional forces, it may still not be sufficiently isolated. For example, the piezoelectric actuator may expand rapidly to open the valve, and after fully opening the valve, the actuator may hit a hard stop. That is, the actuator may overextend and may collide with a rigid stop when the valve reaches a fully open position. Each time the piezoelectric stack hits the hard stop microscopic cracks may form therein and, after repeatedly hitting the hard stop, the device may fail due to fatigue.
The fuel injector of the present disclosure is directed to one or more of the shortcomings set forth above and/or other shortcomings in the art.